The disclosure of Japanese Patent Application No. 11-281071 filed on Oct. 1, 1999, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a steering unit for a vehicle, especially to a rack bar, which is a component member of a power steering unit, and the manufacturing method thereof.
2. Description of Related Art
A rack bar having: (1) a rod portion which fits slidably into an oil seal attached to a cylinder portion of a power cylinder, and (2) a rack portion formed with the rod portion through a connecting portion which meshes with a pinion thereof is well known in the art. Also known is a technology for quenching the rack portion for enhancing the strength of the rack portion (see Japanese Laid-Open Patent Application No. 6-200320).
When quenching not only the rack portion but also the rod portion, quenching needs to be performed at different conditions for the rod portion and the rack portion, because they differ in structure, thickness, and the like. In this case, an unquenched boundary region (a quenching boundary region which is not completely quenched resulting in an insufficient enhancement in strength) may be formed between the quenched regions of the rod portion and the rack portion.
When bending force is input to the rack bar through a tire, the bending moment acts on the rack bar. The bending moment is at a maximum at a meshing portion between the rack bar and the pinion at the rack portion, and becomes increasingly smaller farther away from the meshing portion in the axial direction.
Accordingly, bending strength may be of a concern if the above-mentioned unquenched boundary region (the weakest portion of the rack bar) is made the portion which comes close (e.g., the rack side end portion of connecting portion) to the meshing portion between the pinion during the course of displacement of the rack bar in the axial direction (that is, when turning a steering wheel).
In view of above-mentioned drawbacks, it is an object of the invention to reduce the bending moment which acts on the unquenched boundary region when a bending force is input from outside and to improve the bending strength of the rack bar so as to maintain its durability.
A rack bar for a power steering unit according to one aspect of the invention includes a rod portion that is slidable into an oil seal attached to a cylinder portion of a power cylinder, a connecting portion provided at one end of the rod portion and a rack portion which is connected to the rod portion through the connecting portion and which has racks for meshing with a pinion. The rack bar also includes a first quenched region formed on at least a portion of the rod portion, and a second quenched region formed continuously on at least a portion of the rack portion and the connecting portion.
Therefore, it becomes possible to reduce the bending moment which acts on the unquenched boundary region when a bending force is input from outside and to improve the bending strength of the rack bar so as to maintain its durability.